A bidirectional synchronous converter, which can also be called a “buck-boost” converter, is known from DE 696 34 709 T2. A circuit is also known which can be used to sense the current via the power inductor of the switch converter. Additional conventional circuits and methods of sensing current are mentioned in the document. The document also addresses problems with these known methods and circuits for sensing the current by the switch converter.
The conventional circuits and methods are based on two different principles, specifically the sensing of the current via a current sensing resistor, and the sensing of the current by means of a transformer and a current sensing resistor in a secondary circuit of the transformer.
The disadvantages of the two principles will now be addressed briefly.
When a current sensing resistor is used, it is advantageous that only currents with frequencies which are a great deal smaller than the limit frequency fg=R/2pi*Lp are sensed, wherein Lp is the parasitic inductance of the current sensing resistor, and R is the resistance of the current sensing resistor. For high currents, low-resistance current sensing resistors are required in order to keep power dissipation low. This results in small limit frequencies.
However, it is advantageous for DC-to-DC converters [to have] switching frequencies, which are also called conversion frequencies, in the range from 100 to 1000 kHz. Only in this case is it possible to obtain satisfactory degrees of efficiency and constructed sizes at a reasonable cost. Otherwise, expensive throttles and complex filter elements are necessary in order to achieve the necessary protection from electromagnetic radiation (EMC).
The dependence of the current sensing resistor on frequency can be compensated with filter circuits if the parasitic inductance is known and is specified by the manufacturer. However, this is not generally the case. Therefore, it is generally not possible to make this compensation.
As an alternative, in order to achieve high limit frequencies, high-resistance current sensing resistors could be used. However, this results in high power dissipation. An additional cooling is then also necessary.
The sensing of current by means of a transformer and a current sensing resistor in the secondary circuit, which is particularly also disclosed in DE 696 34 709 T2, makes it necessary to acquire an expensive transformer.
The publication “LM5116 Wide Range Synchronous Buck Controller” from Dec. 8, 2008, published by the National Semiconductor Corporation, discloses a circuit (page 11 of the publication) by means of which the current is not sensed but rather is emulated by a power inductor of a DC-to-DC converter, which is a synchronous DC-to-DC converter. A current sensing resistor is arranged in the DC-to-DC converter and is in series with the power inductor, at least chronologically, wherein the coil current is then fed through the current sensing resistor. The constant component of the voltage which drops across the current sensing resistor is sensed by means of a holding element. The alternating component of the coil current, in contrast, is derived from a voltage difference between the voltage at the input and the voltage at the output of the DC-to-DC converter.
The circuit therefore comprises                a first means for generating a first signal, said means sensing the constant component of the current, wherein the first means has first elements for generating the first signal,        a second means for generating a second signal, said second means emulating the alternating component of the current, and        a third means for splicing the first signal and the second signal into a signal which at least partially emulates the current via the power inductor.        
The sensing of the constant component of the current via the power inductor, by means of the current sensing resistor at the point indicated on page 11 of the named publication, has the following disadvantage:
The current sensing resistor, indicated by RS in the diagram on page 11 of the publication, has (in a simplified view) a parasitic series inductance and a parasitic shunt capacitance. It forms an oscillating circuit which is stimulated by the discontinuous current signal. If the MOSFET, indicated by Q1 in the diagram on page 11 of the publication, is switched off, this has the result that the voltage at the node SW where the power inductor L1 is connected to the two MOSFETs Q1, Q2 reverses sign and becomes more negative than the base potential. At this point, the MOSFET Q2 is forced to switch (the parasitic parallel diode of the MOSFET Q2 starts conducting). The signal at the current sensing resistor demonstrates a self-oscillating behavior according to the final switching times of the MOSFET, the MOSFET capacitances, the switch-on delay of the MOSFET Q2, the storage effect of the lower parasitic diode of the MOSFET Q2, the non-ideal power inductor L1, and the parasitic elements of the current sensing resistor R, itself. The amplitude of this self-oscillation can be many times the desired signal. The working resistance of the current sensing resistor RS must therefore be made as high as possible so that a useable signal-to-noise ratio results. The sampling of the signal by means of a sample and hold circuit is increasingly difficult at smaller resistance values, because the sampling takes a certain amount of time. During this time, the signal being sampled should not vary. In addition, at higher conversion frequencies, the self-oscillating behavior persists longer than the time during which the MOSFET Q2 is conductive. In this case as well, the sampled signal cannot be used.